The invention relates to viscoelastic surfactant fluid systems (VES's). More particularly it relates to an additive package for viscoelastic surfactant fluid systems that increases their thermal stability and shortens the time they take to heal after shearing.
Certain surfactants, when in aqueous solution, form viscoelastic fluids. Such surfactants are termed “viscoelastic surfactants”, or “VES's”. Other components, such as additional VES's, co-surfactants, buffers, acids, solvents, and salts, are optional or necessary (depending upon the specific VES fluid system and the intended use) and perform such functions as increasing the stability (especially thermal stability) or increasing the viscosity of the systems by modifying and/or stabilizing the micelles; all the components together are called a viscoelastic surfactant system or viscoelastic fluid system. Not to be limited by theory, but many viscoelastic surfactant systems form long rod-like or worm-like micelles in aqueous solution. Entanglement of these micelle structures gives viscosity and elasticity to the fluid. For a fluid to have good viscosity and elasticity under given conditions, proper micelles must be formed and proper entanglement is needed. This requires the surfactant's structure to satisfy certain geometric requirements and the micelles to have sufficient length or interconnections for adequate entanglements.
Many chemical additives are known to improve the rheological behavior (greater viscosity and/or greater stability and/or greater brine tolerance and/or lower shear sensitivity and/or faster rehealing if micelles are disrupted, for example by shear). Such materials are typically called co-surfactants, rheology modifiers, or rheology enhancers, etc., and typically are alcohols, organic acids such as carboxylic acids and sulfonic acids, sulfonates, and others. We shall use the term rheology enhancers here. Such materials often have different effects, depending upon their exact composition and concentration, relative to the exact surfactant composition (for example hydrocarbon chain lengths of groups in the surfactant and co-surfactant) and concentration. For example, such materials may be beneficial at some concentrations and harmful (lower viscosity, reduced stability, greater shear sensitivity, longer rehealing times) at others.
In particular, many VES fluid systems exhibit long viscosity recovery times after experiencing prolonged high shear. Slow recovery negatively impacts drag reduction and proppant transport capability, which consequently leads to undesirably high treating pressures and risks of near wellbore screen-outs. Slow recovery of viscosity after shear also means that higher concentrations of viscoelastic surfactants must be used. One way that the expense of higher viscoelastic surfactant concentrations can be offset is to use shear recovery enhancers and/or shear rehealing accelerators that allow the use of lower viscoelastic surfactant concentrations.